This proposal focuses on the analysis of cell-cell interactions in cardinal processes of Drosophila embryonic development, such as determination of neuroblast cell fate, guidance of tracheal cell extension, and differentiation of mesodermal lineages. Each one of these issues is approached by isolation of a receptor tyrosine kinase (RTK) gene which plays a key regulatory role in one these processes. These RTKs are likely to transmit the information determining cell fate from neighboring cells or the extracellular matrix into the cell. Specifically, the role of three Drosophila RTKs will be analyzed in detail by using a variety of cell lineage markers and mutant backgrounds to follow the defects resulting from mutations in the loci encoding them. Their interaction with other known mutations which lead to similar phenotypes will also be tested. A key to understanding the regulation of these signal transduction pathways is the identification of the ligands which trigger each of the respective receptors. Analysis of the structure of the ligands, their expression and processing pattern, and the phenotype of the corresponding mutations is likely to be extremely illuminating. In the case of one receptor, a candidate ligand gene is already available, while in the other two cases fusion proteins containing the extracellular region will be used to isolate clones of the ligands from an expression library. These studies are important to understand crucial aspects of Drosophila embryonic development, and focus on processes which have not been amiable to systematic genetic screens. Some of the principles of development that are expected to be uncovered, for example the mechanism of tracheal cell extension during embryogenesis, are likely to be universal to processes of migration and axonal outgrowth in all multicellular organisms. The Drosophila RTKs are structurally highly similar to their vertebrate homologs, which in many cases were identified as potent oncogenes. Thus, understanding the normal function of RTKs is likely to shed light also on their oncogenic properties. Beyond the conservation of the structure of RTKs, many aspects in their signal transduction pathway appear to be universal. Identification of some of these elements in Drosophila by utilization of genetic screens, may be the most powerful approach to dissect the pathway and identify positive and negative elements which participate in its regulation.